Unstructured supplementary service data (Unstructured Supplementary Service Data, USSD for short) service is a newly emerging, real-time interactive session data service based on a Global System for Mobile Communications (Global System for Mobile Communications, GSM for short) network. For a subscriber identity module (Subscriber Identity Module, SIM for short) card, the USSD service transfers data by using a signaling path on a GSM network and is a new service based on a technology of a short message service system in GSM.
The third generation partnership project (The 3rd Generation Partnership Project, 3GPP for short) standards body has already defined the use of USSD service in legacy second generation (second generation, 2G for short)/third generation (3rd Generation, 3G for short) mobile communications networks, including: USSD service initiated from a mobile terminal side to a network side, and USSD service initiated from a network side to a mobile terminal side.
For example, by entering an unstructured supplementary service operation command in USSD format on a mobile terminal, a subscriber initiates a specific service request to a network side to complete a specific service. The specific services, such as inquiring flight status, train running status, ship time schedule, or weather on a mobile terminal, are all completed by initiating a USSD service request.
While communications networks are evolving from 2G/3G to Long Term Evolution (Long Term Evolution, LTE for short) of 4G, a bearer network used for carrying mobile voice data is also shifting from a traditional switch network and a softswitch network to a voice over LTE (Voice over LTE, VoLTE for short) network. For the VoLTE network, the 3GPP standards body has defined a standard for initiating a USSD service from a mobile terminal side to a network side, but does not yet define a standard for initiating a USSD service from a network side to a mobile terminal side; what's worse, the standards for network-initiated USSD service on legacy 2G/3G networks are not applicable to mobile terminals on the VoLTE network. This compromises the application scope of the prior art, and also hinders the use of VoLTE networking.